This invention relates generally to alloys for use at high temperatures, and more particularly to oxidation-resistant coatings for protecting gas turbine components.
Gas turbine engines, commonly known as "jet engines" when used for propulsion, generate power by drawing in and compressing air, mixing the air with fuel, burning the mixture to create combustion gas, and then expelling the gas outwardly through a turbine. The turbine functions by turning the direction of the combustion gas flow slightly away from an axial path with stationary turbine vane components, and the turned gas flow then impinges upon a series of turbine blade components, which are attached to a rotating turbine wheel. The force of the gas against the turbine blades causes the turbine wheel to turn rapidly. Turbine efficiency increases with increasing nominal operating temperature, but the ability of the turbine to operate at increasingly great temperatures is limited by the ability of the turbine vanes and blades to withstand the heat, oxidation and corrosion effects of the impinging hot gas stream and still maintain sufficient mechanical strength. Thus, there exists a continuing need to find material systems for use in components that will function satisfactorily in gas turbines at higher temperatures and stresses.
One approach to providing improved turbine components is to fabricate a strong, stable substrate having the shape of the component, and cover the substrate with a thin protective coating that resists the oxidation and corrosion effects of the hot combustion gas stream. The underlying substrates, usually nickel-base or cobalt-base superalloy compositions, were formed by forging or simple casting procedures at one time, but improved performance results from use of directional solidification or directional recrystallization procedures. Even greater operating temperatures are possible by casting the substrate as a single crystal having no grain boundaries which might cause premature failure, and with the single crystal oriented for best performance at the operating temperature.
A coating of about 0.001-0.010 inches thickness is usually applied to protect the substrate, through formation of an adherent oxide such as aluminum oxide that resists the oxidizing effects of the hot combustion gas stream. Other elements present in the coating resist hot salt corrosion and contribute to the ability of the protective oxide scale coating to adhere to the coating through many cycles of gas turbine startup and shut-down.
As nominal gas turbine operating temperatures are raised through development of improved substrate materials, it is critical that the performance of the coating and the substrate be optimized to achieve the greatest overall performance. Single crystal superalloy components are particularly attractive for use at high turbine temperatures, but to achieve improved high temperature performance the single crystal substrate chemical compositions have been modified in several subtle but important ways. As a result of these modifications, the coatings used in previous component protective systems do not provide the optimum performance when used with substrates cast as single crystals.
There therefore exists a need for an improved coating to be used to protect turbine components. In particular, there is a need for an improved coating which contributes to the optimization of performance of single crystal superalloy turbine blades and vanes, but can also be used with polycrystalline components. The present invention fulfills this need and further provides related advantages.